Microwave unit for thermographic printing

ABSTRACT

A thermography machine for relief printing equipped with a microwave device comprising: a generator FIG. 2 13 whose transmission frequency may be 2.45 GHz feeding an applicator 14 scattering the microwave energy over the powder image to be melted, a movable short-circuit 15 adjusting the length of the applicator 14 to make it resonate at the frequency of the microwave generator (for example 2.45 GHz), a nonreturn device arranged between the generator 13 and the applicator 14 and an electronic device for automatic tuning of the applicator 14.

This application is a continuation of application Ser. No. 076,137,filed July 21, 1987 and now abandoned.

The present invention relates to microwave transmitters intended to beapplied to thermography machines for relief printing.

Thermography or relief printing is a known process. It makes itpossible, starting with a typographical print, offset or otherwise, toproduce a relief print imitating plate printing or stamping. .The reliefconversion is simple and consists in sprinkling a freshly printed papersheet with a powder which has the property of melting under the effectof heat and of forming a relief film after melting.

Only the moist ink retains the powder and the excess is continuouslyrecovered. The "powdered" printed matter then passes through a tunneloven where the printed and powdered substrate needs to withstand a hightemperature of the order of 150° to permit the powder to melt and loseenough viscosity to spread and form a smooth relief film. At the exit ofthe tunnel oven a jet of cold air cools the paper and solidifies theviscous relief film to prevent the sheets from sticking together.

Automatic conversion into relief is carried out as follows: on leavingthe printing press, the paper is received directly on conveyor belts andpasses, in succession, under the powdering unit, inside the tunnel ovenand over a final conveyor, to cool, before being received in a deliverybox. The powders employed, which are glossy or matt, are transparent andretain their hues in print colors. Pigmented powders, on the other hand,produce a relief which corresponds to their pigment, whatever the printcolor.

The particle size of the powder employed determines the thickness of therelief layer. The coarser the powder, the higher the relief.

The machines which are in use at present restrict the possibilities ofthis process, because they have a certain number of defects, the chiefof which are the following:

high cost of the relief conversion, due to a very high energyconsumption. This defect is particularly noticeable in the case ofhigh-weight cards such as cardboard,

more or less pronounced yellowing, depending on the type of paper orcard,

partial destruction of the substrate fiber, making subsequent treatmentssuch as folding, box forming, and the like very difficult,

erratic losses in the size of the substrate, generally resulting in itsshrinkage. Apart from other defects, this defect practically rules outthe use of this process for printed matter intended for datamicroprocessing. In fact, the majority of printed matter of this type isintended to form sets of a certain number of superposed sheets, each ofwhich works in register with all of those forming the set,

costly cooling of the substrate, due to the fact that the conveyors mustbe long, to enable the fibrous mass, which is an excellent heatinsulator, to remove a considerable part of the heat energy taken upwhen the temperature of the printed substrate is raised, and

a machine which is very bulky because of the abovementioned defects.

At the present time, melting of the powder is carried out by means oftunnel ovens, most of which are made of heating elements which radiateinfrared and are in most cases fed by electrical or gas systems. Thewavelength of the radiation is generally between 2 and 10 μm. Melting ofthe powder is produced by the combination of infrared radiation and theraising of the air temperature inside the tunnel oven, which goes up toabout 350° centigrade, depending on the oven.

The paperboard industry is generally concerned with printing sheets ofcardboard, starting with a single format of 50×70 centimeters orquadruple format of 100×140 centimeters in basic weights of the order of330 grams per square meter. The printing rates are approximately 6000copies per hour.

To give an indication, the relief conversion in a single format ofprinted matter of this kind requires a machine whose generalcharacteristics are as follows:

width of the tunnel oven passage: 75 centimeters,

power absorbed: 90 to 100 kilowatts,

length of the tunnel oven: 4 meters, and

length of the cooling conveyors: 4 to 5 meters.

In the case of a quadruple format of 100×140 centimeters, the width ofthe passage would be doubled and the oven power would be substantiallyquadrupled.

Summary of the Invention

The present invention is aimed at overcoming all the abovementioneddefects and chiefly at considerably improving the energy balance of thisrelief conversion by replacing the conventional tunnel ovens with amicrowave device whose performance enables the energy consumption to bedivided by a factor of more than four in the case of 50×70 cm formatsand by more than eight in the case of the quadruple formats, because theconsumption is substantially the same over passage widths of between 50and 150 cm.

Furthermore, since the heating of the substrate is, broadly, divided bya factor of three, the length of the cooling conveyors isproportionately shortened.

By way of example, without implying any limitation, a type of microwavedevice fitted to a thermography machine is described with reference tothe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art thermography machine;

FIG. 2 is a perspective view of a thermography machine constructed inaccordance with this invention;

FIG. 3 is an enlarged perspective view illustrating one of the microwaveunits for the thermography machine of FIG. 2.

FIG. 1 shows a front view of a conventional thermography machineequipped with a conventional tunnel oven 10, containing electricalheating elements or gas burners. In general outline, these machinesconsist of a supporting structure 1, FIG. 1, carrying all the conveyorsfor feeding 2 the printed matter, for powdering 3, for melting thepowder 4 and for cooling 5. The printed matter 6 is received by theprinting press 7 on the feed conveyor 2 and then on the powderingconveyor 3, where a layer of powder is discharged onto the printedmatter by the powder trough 8. The cyclone 9 sucks away and recycles theexcess powder which is not retained by the moist ink. The printed matterthen passes onto the melting conveyor 4 inside the tunnel oven 10 andonto the conveyor for cooling by means of air driven by a turbine 11A,blowing inside the perforated casing 11. A delivery box 12 completesthis assembly.

FIG. 2 shows a front view of the same machine, in which the tunnel oven10 has been replaced with the microwave device 13-14-15, and the coolercasing 11 and its conveyor 5 have been shortened by virtue of the factthat a melting process using microwaves considerably reduces the heatingof the printed matter.

FIG. 3 shows a more detailed perspective view of a microwave device,FIG. 2 and 3, 13-14-15, characterized in that it comprises aconventional microwave generator 13, FIG. 3, whose transmissionfrequency may be 2.45 GHz (a frequency especially allocated toindustrial applications) and which feeds an applicator or wave guide 14employed to scatter the microwave energy over the product to be melted 6which is placed on the conveyor 4. The other end of the applicator endsin a movable short-circuit or end plate 15 so that the length of theapplicator can be adjusted to make it resonate at the frequency of themicrowave generator (for example 2.45 GHz). The choice of the shape andof the size of the applicator 14 permits the excitation of a particularmode of wave propagation which is such that the coupling between thewave and the product to be melted is optimized. The generator set 13,the applicator 14 and the movable short-circuit 15 forming the microwavedevice are fastened to the structure 1.

By way of an example which does not imply any limitation, an applicator14 whose cross-section is a rectangle of 8.6×13 centimeters will beadvantageously chosen, which allows it to be coupled to a standard guide8.6×4.3 centimeters in size, by means of an iris whose coupling holearea permits an optimum coupling of the applicator 14 to the generator13 when the applicator resonates. The electrical field in the applicator14 is parallel to the side whose size is 13 centimeters. An automatictuning device may be provided to permit automatic monitoring of theresonance tuning of the applicator 14.

When the applicator 14 does not contain any product to be melted, itsresonance frequency is different from that of the generator 13, which isfixed (for example 2.45 GHz) and the incident wave may be reflectedstrongly and may damage the generator. A nonreturn (circulator) devicemay then be inserted between the generator 13 and the applicator 14, ora device for detecting the presence or absence of the product to bemelted in the applicator 14 may be provided. In the absence of theproduct, an automatic system will be able to reduce the transmissionpower of the generator 13, and this will avoid the use of a nonreturnprotection circulator. All of these protection means are of knownelements. The combination in series of several elementary microwave setssuch as that of FIG. 3 forms the microwave device intended for meltinghot-melt powders. The number of elementary sets which are combined inseries obviously depends on the processing speed which is aimed at. Theposition of each elementary set relative to the neighboring sets ischosen so that the complete microwave device provides a practicallyhomogeneous treatment over the entire width of the substrate to beprinted. The use of a microwave device of this kind for melting hot-meltpowders has shown that it enables the following advantages to beobtained:

heating of the substrate is much more selective than when an infraredtreatment is employed, and is much lower overall,

wave coupling to the support and to the powder is proportionately betterthe higher the basic weight of the substrate,

treatment over the 1.40-meter width presents no problem and consumesenergy of the same order of magnitude as in the case of a substratewhich is much less wide (for example 70 centimeters),

treatment of 50 meters per minute of a substrate of high basic weightand 1.40 meters wide would require approximately 15 kW microwavesinstead of approximately 200 kW using infrared,

microwaves are switched on and off instantaneously.

It is therefore possible to bring the microwaves into operation onlyduring the passage of the product through the device, and thisconstitutes a major energy saving and limits the heating of theapplicator, and

elimination of the risk of fire in the event of stoppage of thesubstrate due to a failure of the conveyor belt or blockage of thedevice.

While the invention has been shown in only one of its forms, it shouldbe apparent to those skilled in the art that it is not so limited, butis susceptible to various changes without departing from the scope ofthe invention.

I claim:
 1. An apparatus for producing relief printing, comprising incombination:a printing press for printing images on printed matter witha liquid ink; conveyor means including a conveyor belt for conveying theprinted matter on the belt from the printing press while ink on theprinted matter is still wet; powdering means including a trough mountedover the conveyor belt for distributing a thermographic powder from thetrough onto the wet ink on the printed matter and for sucking away fromthe printed matter and recycling excess powder; microwave meansincluding a microwave generator and a wave guide for applying microwaveenergy to the powder contained on the printed matter for melting thepowder, the wave guide being mounted over the belt; cooling meansincluding a tunnel mounted over the belt and a fan for discharging aironto the printed matter to cool the powder after it has melted; andmeans for sensing the presence of printed matter under the wave guide,and for automatically reducing the transmission of powder from themicrowave generator in the absence of printed matter.
 2. An apparatusfor producing relief printing, comprising in combination:a printingpress for printing images on printed matter with a liquid ink; conveyormeans including a conveyor belt for conveying the printed matter on thebelt from the printing press while ink on the printed matter is stillwet; powdering means including a trough mounted over the conveyor beltfor distributing a thermographic powder from the trough onto the wet inkon the printed matter and for sucking away from the printed matter andrecycling excess powder; a microwave generator; a wave guide having oneend connected to the microwave generator for distributing microwaveenergy from the microwave generator to the powder contained on theprinted matter for melting the powder, the wave guide extending over thebelt perpendicular to the belt; a slidable end plate means mounted to anend of the wave guide opposite the end connected to the generator, theend plate means being slidable in a direction perpendicular to the beltfor varying the length of the wave guide and thus the resonance of thewave guide; the wave guide being unobstructed from the generator to theend plate means for directing microwave energy transversely across theprinted matter and transversely through the powder; cooling meansincluding a tunnel mounted over the belt and a fan for discharging aironto the printed matter to cool the powder after it has melted; andmeans for sensing the presence of printed matter under the wave guide,and for automatically reducing the transmission of power from themicrowave generator in the absence of printed matter.